Alaska Science Center
Disentangling effects of growth and nutritional status on seabird stable isotope ratios
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Full Publication: http://www.springerlink.com/content/d0417385hw550410
Product Type: Journal Article
Authors: Sears, J., S. A. Hatch, and D. M. O'Brien
A growing number of studies suggest that an individual’s physiology affects its carbon and nitrogen stable isotope signatures, obscuring a signal often assumed to be only a reflection of diet and foraging location. We examined effects of growth and moderate food restriction on red blood cell (RBC) and feather ä15N and ä13C in rhinoceros auklet chicks Cerorhinca monocerat), a piscivorous seabird. Chicks were reared in captivity and fed either control (75 g/day; n = 7) or ~40% restricted (40 g/day; n = 6) amounts of high quality forage fish. We quantified effects of growth on isotopic fractionation by comparing ä15N and ä13C in control chicks to those of captive, non-growing subadult auklets (n = 11) fed the same diet. To estimate natural levels of isotopic variation, we also collected blood from a random sample of free-living rhinoceros auklet adults and chicks in the Gulf of Alaska (n = 15 for each), as well as adult feather samples (n = 13). In the captive experiment, moderate food restriction caused significant depletion in ä15N of both RBCs and feathers in treatment chicks compared to control chicks. Growth also induced depletion in RBC ä15N, with chicks exhibiting lower ä15N when they were growing the fastest. As growth slowed, ä15N increased, resulting in an overall pattern of enrichment over the course of the nestling period. Combined effects of growth and restriction depleted ä15N in chick RBCs by 0.92‰. We propose that increased nitrogen-use efficiency is responsible for 15N depletion in both growing and food-restricted chicks. ä15N values in RBCs of free-ranging auklets fell within a range of only 1.03‰, while feather ä15N varied widely. Together, our captive and field results suggest that both growth and moderate food restriction can affect stable isotope ratios in an ecologically meaningful way in RBCs although not feathers due to greater natural variability in this tissue.